Numerous pipe inspection pigs are in existence and have been used in connection with non-destructive inspection of pipelines for gaseous or liquid materials, such as natural gas, liquid hydrocarbons, or water.
Most underground pipelines are made out of ferromagnetic steel and for that reason, inspection devices such as eddy current detectors, as may be used in inspection of non-ferromagnetic tubing, such as stainless steel or other types of tubing used in heat exchangers and the like are not particularly useful in connection with underground steel pipelines, such as chemical pipelines, liquid hydrocarbon pipelines, gas pipelines, water or sewage pipes, and the like.
Various methods of detecting flaws or defects from the inside of a pipe or pipeline have been attempted with varying degrees of success. Ferromagnetic induction devices have been used as disclosed in U.S. Pat. No. 4,742,298. This invention was directed to determining the presence and the magnitude of surface flaws and to overcoming difficulties encountered in determining the presence and the magnitude of surface flaws in a pipe. The solution proposed was to use a cylindrical primary alternating current coil which is coaxially aligned with the pipe to generate a high frequency AC magnetic field in the pipeline, a multiple cylindrical secondary AC sensing coil where arranged at prescribed intervals in a circumferential direction around the interior of the pipe, each secondary coil having an axis parallel to the axis of the primary coil. The AC voltage sensed at each secondary coil is set to be proportional to the density of a parallel component of magnetic flux caused by the AC magnetic field generator.
Eddy current sensing probes have also been used primarily in connection with non-destructive inspection and testing of relatively thin-walled tubing which is not ferromagnetic material. Such tubing does exist in steam generators and heating exchangers having been the primary focus of eddy current probes as disclosed in U.S. Pat. No. 4,851,773 which discloses a single direction rotating head profilometer. One embodiment of that device discloses an electromechanical eddy current probe having a rotatable sensing head for sensing the wall thickness and for locating local defects in a tube or conduit through which it is passed. Basically, the mechanical profilometer probe was designed to detect dents in the interior surface of steam generator tubes. The position of the rotating head is varied along the length of the tubing being inspected as the probe is drawn through the tubing with a cable.
Another eddy current probe is disclosed in U.S. Pat. No. 4,952,875 in which a plurality of pairs of diametrically opposed sensing coils are alternatingly staggered along the longitudinal axis of the test sensor to give complete coverage of the interior pipe surface and are further permitted to move in and out to accommodate the size differences or constrictions in the pipeline. However, the sensor probe is intended to move longitudinally through the pipeline.
Also, U.S. Pat. No. 5,068,608 discloses multiple coil eddy current probe system and an eddy current probe is disclosed in which a defect is first detected when the probe is positioned adjacent the defect and a series of axially spaced probes are activated to sense and detect the extremities of a crack or other discontinuity. Generally, eddy current probes have not been particularly successful with respect to underground pipelines constructed of steel or other ferromagnetic materials and having pipeline walls with thicknesses substantially greater than the normal eddy current penetration depth. However, one attempt to provide an eddy current probe or ferromagnetic pipeline flaw detection was disclosed in U.S. Pat. No. 4,107,605. There is no indication of the usefulness of such probes in connection with determining longitudinal cracks which are parallel to the direction of movement of the probe assembly.
The most popular and currently most useful sensors for ferromagnetic pipeline inspection have been magnetic flux generators and magnetic flux leakage sensors which are positioned circumferentially around an inspection pig which is moved longitudinally through the pipeline. Examples of such sensors are disclosed in U.S. Pat. Nos. 4,105,972, 4,310,796, 4,444,777 and 4,458,601. The operation of such magnetic flux detection probes is described in U.S. Pat. No. 4,789,827 in connection with a magnetic flux detection probe in which the sensors are intentionally spaced at different radial distances or spaced different distances from the interior pipe surface in an effort to obtain greater accuracy with respect to the location of the flaw or defect on the inside or the outside of the pipe wall.
Some attempts have been made to detect defects at different angular orientations in connection with testing and inspecting pipes as they are being manufactured. U.S. Pat. No. 3,906,357 discloses an exterior pipe testing device in which there are two external sensor sections, one having a plurality of fixed sensing shoes circumferentially spaced around the pipe to be inspected which depends upon linear movement of the pipe therethrough for detecting flaws or defects primarily oriented circumferentially around the pipe. A second inspection unit is provided which has a pair of opposed magnetic sensing shoes which is rotated rapidly around the outside of the pipe to be inspected in an effort to detect longitudinal cracks which might otherwise go unnoticed with the fixed shoe sensing unit. Complex circuitry is used to coordinate the sensor input from each of the sensing units with a rotating magnetic pulse generator geared to the linear motion of the pipe being manufactured. A purpose of this device is to actuate one or more spray cans at the linear and the circumferential position where a manufacturing flaw is detected either by the linear inspection unit or the rotary inspection unit. Application of such a testing device to on-site underground pipelines has not been demonstrated.
Another exterior pipe testing device has been disclosed in U.S. Pat. No. 4,439,730, in which pairs of north and south poles of magnets are held adjacent to the exterior wall of a pipe at uniformly spaced apart positions circumferentially around the pipe. The north and south poles are positioned between the north and south poles of longitudinally spaced apart circular magnets around the pipe. The circumferential spaced apart magnets are rotated at a high rate of speed so that orthogonically directed resultant magnetic field is produced on opposite sides of the pipe between the north and south pole of the rotating magnets. Pairs of flux detectors are interposed on opposite sides of the rotating magnet. The magnets are rotated at a sufficiently high rate of speed relative to the longitudinal motion of the pipe since the flux field interruptions in the same incremental area of the pipe. Again, complex circuitry is required in order to coordinate the sensor input from each of the sensing units because of the high rotational speed (320 revolutions per minute in the example set forth in '730) in order to keep track of the sampled signals from the two overlapping sensors and further, to coordinate them to a longitudinal position along the pipe. At a longitudinal travelling speed of 80 feet per minute as set forth in the example, the device must make four complete revolutions during every one foot of travel, which is consistent with the sensor field slightly over three inches long, so that 100% of the pipe surface can be covered. Such a device is not considered practical for internal inspection of existing underground pipelines. Potentially, the rate of rotation may not be achievable for internal pipe inspection devices.
Pipeline flaw detectors for use inside of existing pipelines have also provided rotary mechanisms for rotating sensing shoes helically through the pipeline as the detector is moved linearly therealong. One such device is disclosed in U.S. Pat. No. 3,238,448 which, upon detecting a flaw, actuates a strong electromagnet to magnetize the corresponding portion of the pipeline so that the position of the defect can be detected from aboveground with magnetic sensors. This device rotates two opposed search units in a single direction such that only very large flaws can be accurately detected and locating any such detected flaws is dependent upon a second careful searching action for the magnetized pipe section from aboveground.
Another pipeline inspection apparatus is disclosed in U.S. Pat. No. 4,072,894 which produces a circumferentially directed magnetic flux field as flux leakage detection sensors are resiliently held against the pipe wall surface and helically moved through the pipe to pass transversely across any longitudinally extending anomalies in the pipe wall. This device produces only a circumferentially directed magnetic flux and helical movement of the sensing probes in only one direction.
One of the most popular and currently the most widely used state-of-the-art internal magnetic flux gas pipe inspection devices comprises a pipeline pig which has sealing cups around the exterior perimeter to both center the apparatus and to drive it by differential gas pressure along the pipeline. A magnetic flux is generated by multiple circumferentially spaced magnets with north and south poles axially spaced apart and a magnetic flux sensor interposed therebetween. In operation, the pig travels linearly through the pipeline and sensory input data from each sensor is recorded as a function of distance of travel. When a defect, void, or other anomaly in the pipe is indicated by sensing an interruption of a smooth longitudinal magnetic flux, then such an anomaly is recorded on a graph as a function of time or distance. A major drawback of this device is that the longitudinal, or axially aligned, magnetic flux cannot always detect longitudinal voids or defects such as a uniform deterioration along a continuous welded seam of the pipeline. Resolution is determined by the size of the multiple sensor unit. A second set of circumferentially positioned magnetic flux generators and flux leakage sensors can be positioned at a small staggered distance with respect to the first set so that the space between the flux generator and sensor shoes is covered by the second set of sensors. Still, minor disturbances at the start of a longitudinal defect and at a distant end of the longitudinal defect may go unnoticed on a graph.
The best resolution available is approximately limited by the size of the gap between the sensors. Often, one or more of the multiple sensors may fail during a run several miles through a pipeline, which may give an entire line of approximately one to three inches wide in which no discontinuities would be detected along the length of the pipe. In order to reduce some of this risk, the pigs are often rotated at up to about a 1.degree. angle, which amounts to about one revolution per 1,000 linear feet. The magnetic flux is still linearly aligned in the axial direction and the small amount of rotation, if any, is so small that longitudinal voids continue to be substantially undetectable.